1. Field of the Invention
This invention relates to testing of telephone lines using time-domain, reflectometry.
2. Description of the Prior Art
A typical telephone subscriber receives telephony services over a pair of copper wires at frequencies below 4 kHz. Recent transmission technology, such as digital subscriber line (DSL), has expanded the possible use of copper wires to enable high speed data transmissions using frequencies up to and beyond 1 MHz. However, installation practices and defects or anomalies in copper wire pairs can limit or distort the transmission carrying capacity thereof. To test for possible problems in copper wire pairs, a single-ended test is desired to avoid or minimize the time and expense of dispatching repair personnel to the far end of a copper wire pair for a dual-ended test.
Time-domain reflectometry (TDR) is a well-known and generally available technique that can be utilized to identify problems associated with copper wire pairs. TDR apparatus and methods are disclosed generally in U.S. Pat. No. 5,121,420 to Marr et al.; U.S. Pat. No. 5,369,366 to Piesinger; 5,461,318 to Borchert et al.; 5,521,512 to Hulina; 5,530,365 to Lefeldt; and 5,650,728 to Rhein et al. However, traditional TDR techniques have several limitations. Specifically, present TDR techniques include transmitting an electrical pulse down the copper wire pair and measuring the time to receive a return pulse. This return pulse occurs when the transmitted pulse encounters a change in impedance of the copper wire pair due to some discontinuity therein. Common causes of discontinuities in the copper wire pair include: splices where different copper wire pairs are joined together; moisture on or around the copper wire pair; connection of bridge taps to the copper wire pair; or terminations, such as telephones, that may be connected to the copper wire pair.
Conventional TDR is limited by the energy content of the pulse and the frequency dispersion of the pulse as it travels along the length of the copper wire pair and back. These limitations include: technical difficulty in coupling all of the source TDR energy pulse to the copper wire pair; very low return signal levels due to losses associated with round trip pulse transmission along the copper wire pair; xe2x80x9csmearingxe2x80x9d of return pulses due to multiple reflections in both directions of pulse travel and a low signal-to-noise ratio (SNR) on a lossy copper wire pair.
It is, therefore, an object of the present invention to provide a method of performing time-domain reflectometry that avoids the above problems and others associated with conventional TDR techniques. It is an object of the present invention to provide an apparatus for performing the method. Still other objects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.
Accordingly, I have invented a delay-line time-domain reflectometer that includes a charge generator for selectively charging a line capacitor formed by the physical relation of at least two electrically conductive leads to a predetermined DC voltage. A line clamp selectively connects the leads together and a return detector receives a discharge pulse produced by discharge of the line capacitor in response to the line clamp connecting the leads together. The return detector also detects an end-of-line (EOL) pulse and/or a bridged-tap (BT) pulse superimposed on the discharge pulse and outputs an analog return signal that is a function of the discharge pulse, the EOL pulse and/or the BT pulse. The EOL pulse is produced by interaction between the discharge pulse and a terminal end of the leads and the BT pulse is produced by interaction between the discharge pulse and a BT connected to the leads.
Preferably, the charge generator charges the line capacitor as a function of the length of the conductive leads. More specifically, the charge generator charges the line capacitor as a function of the section of the length of the conductive leads being tested. For example, the charge generator charges the line capacitor to a first voltage which produces for a first section of the conductive leads a discharge pulse having an EOL pulse and/or a BT pulse superimposed thereon having sufficient signal-to-noise ratio (SNR) to enable detection thereof by the return detector. For testing a second section of the line, the charge generator charges the line capacitor to a second voltage which produces a discharge pulse having an EOL pulse and/or a BT pulse superimposed thereon having sufficient SNR to enable detection thereof by the return detector. Similar comments apply in respect of other voltages utilized for testing other sections of the conductive leads.
A charging resistor can be included for limiting a charging current utilized to charge the line capacitor and isolated the charge generator from the line capacitor when the line clamp connects the leads together.
A controller can selectively control the operation of the charge generator and the line clamp to charge the line capacitor to a predetermined DC voltage and to connect the leads together, respectively. The controller can also process the analog return signal to determine a distance between the delay-line time-domain reflectometer and a terminal end of the leads, a distance between the delay-line time-domain reflectometer a BT and/or a distance between the terminal end of the BT and its connection to the leads.
A digital-to-analog converter (DAC) can be connected to receive and convert digital waveform data into an analog signal which is supplied to the charge generator for use thereby to charge the line capacitor. An analog-to-digital converter (ADC) can convert the analog return signal into digital return data. The controller can supply the digital waveform data to the DAC and can receive the digital return data from the ADC.
The controller can include a programmable logic device (PLD) responsive to a control request signal for supplying the digital waveform data to the DAC and for generating one or more control signals which control the operation of the DAC and the ADC. A digital signal processor (DSP) can be connected for supplying the control request signal to the PLD and for receiving the digital return data from the ADC. A host processor can supply a test request to the DSP and can receive therefrom a test result which is a function of the digital return data.
A rail supply can be connected to receive DC power from an external source thereof and to convert the received DC power into one or more electrical potentials for use by the return detector. Preferably, the rail supply is configured to isolate the return detector from the external source of DC power during receipt of the discharge pulse by the return detector.
I have also invented a method of time-domain reflectometry comprising the steps of charging a line capacitor formed by the physical relation of electrically conductive leads of a telephone line to a predetermined DC voltage. The leads are connected together and a discharge pulse produced by discharge of the line capacitor in response to connecting the leads together can be received. An EOL pulse and/or a BT pulse superimposed on the discharge pulse can be detected. The EOL pulse is produced by interaction between the discharge pulse and a terminal end of the leads and a BT pulse is produced by interaction between the discharge pulse and a BT connected to the leads. From the discharge pulse, the EOL pulse and/or the BT pulse, a location of a terminal end of the leads, a location of the BT connection to the leads and/or a location of a terminal end of the BT relative to its connection to the leads can be determined.
Lastly, I have invented an apparatus for testing a telephone line. The apparatus includes a charge generator for charging the telephone line to a predetermined DC voltage. A switch is provided for connecting together leads of the telephone line when the telephone line is charged to the predetermined DC voltage. A return detector processes a discharge pulse on the telephone line to produce a return signal. The discharge pulse is produced in response to connecting the leads of the telephone line together when the line capacitor is charged to the predetermined DC voltage. A means for processing is provided for processing the return signal to determine one or more characteristics of the telephone line.
The means for processing can include a first converter for converting the return signal into return data and a controller for processing the return data to determine the one or more characteristics of the telephone line. The controller synchronizes the operation of the charge generator and the switch to charge the line capacitor and to connect the leads together, respectively. A second converter can be provided for converting waveform data into a signal which is utilized to control the charge generator to charge the telephone line. The controller can supply the waveform data to the second converter.